Copper containing materials are widely used in industry as catalysts and sorbents. The water shift reaction in which carbon monoxide is reacted in presence of steam to make carbon dioxide and hydrogen as well as the synthesis of methanol and higher alcohols are among the most practiced catalytic processes nowadays. Both processes employ copper oxide based mixed oxide catalysts.
Copper-containing sorbents play a major role in the removal of contaminants, such as sulfur compounds and metal hydrides, from gas and liquid streams. One new use for such sorbents involve the on-board reforming of gasoline to produce hydrogen for polymer electrolyte fuel cells (PEFC). The hydrogen feed to a PEFC must be purified to less than 50 parts per billion parts volume of hydrogen sulfide due to the deleterious effects to the fuel cell of exposure to sulfur compounds.
Copper oxide (CuO) normally is subject to reduction reactions upon being heated but it also can be reduced even at ambient temperatures in ultraviolet light or in the presence of photochemically generated atomic hydrogen.
The use of CuO on a support that can be reduced at relatively low temperatures is considered to be an asset for some applications where it is important to preserve high dispersion of the copper metal. According to U.S. Pat. No. 4,863,894, highly dispersed copper metal particles are produced when co-precipitated copper-zinc-aluminum basic carbonates are reduced with molecular hydrogen without preliminary heating of the carbonates to temperatures above 200° C. to produce the mixed oxides.
However, easily reducible CuO is disadvantageous in some important applications. The removal of hydrogen sulfide (H2S) from gas streams at elevated temperatures is based on the reaction of CuO with H2S. Thermodynamic analysis shows that this reaction results in a low equilibrium concentration of H2S in the product gas even at temperatures in excess of 300° C. The residual H2S concentration in the product gas is much higher (which is undesirable) when the CuO reduces to Cu metal in the course of the process since reaction (1) is less favored than the CuO sulfidation to CuS.2Cu+H2S=Cu2S+H2  (1)Therefore, a reduction resistant CuO sorbent would be more suitable for exhaustive removal of H2S from synthesis gas assuring a purity of the H2 product that is sufficient for fuel cell (PEFC) applications.
Copper oxide containing sorbents are well suited for removal of arsine and phosphine from waste gases released in the manufacture of semiconductors. Unfortunately, these gases often contain hydrogen, which in prior art copper oxide sorbents has triggered the reduction of the copper oxide. The resulting copper metal is less suitable as a scavenger for arsine and phosphine. A further detriment to the reduction process is that heat is liberated which may result in runaway reactions and other safety concerns in the process. These facts are other reasons that it would be advantageous to have a CuO containing scavenger that has an improved resistance towards reduction.
Combinations of CuO with other metal oxides are known to retard reduction of CuO. However, this is an expensive option that lacks efficiency due to performance loss caused by a decline of the surface area and the lack of availability of the CuO active component. The known approaches to reduce the reducibility of the supported CuO materials are based on combinations with other metal oxides such as Cr2O3. The disadvantages of the approach of using several metal oxides are that it complicates the manufacturing of the sorbent because of the need of additional components, production steps and high temperature to prepare the mixed oxides phase. As a result, the surface area and dispersion of the active component strongly diminish, which leads to performance loss. Moreover, the admixed oxides are more expensive than the basic CuO component which leads to an increase in the sorbent's overall production cost.
The present invention comprises a new method to increase the resistance toward reduction of CuO powder and that of CuO supported on a carrier, such as alumina. Addition of a small amount of a salt, such as sodium chloride (NaCl) to the basic copper carbonate (CuCO3.Cu(OH)2) precursor, followed by calcination at about 400° C. to convert the carbonate to the oxide, has been found to significantly decrease the reducibility of the final material. An increase of the calcination temperature of BCC beyond the temperature needed for a complete BCC decomposition also has a positive effect on CuO resistance towards reduction, especially in the presence of Cl.
Surprisingly, it has now been found that calcination of intimately mixed solid mixtures of basic copper carbonate (abbreviated herein as “BCC”) and NaCl powder led to a CuO material that was more difficult to reduce than the one prepared from BCC in absence of any salt powder.